Make that “energetically less costly,” in science-speak, and you have the conclusion of researchers who are proposing a likely reason for our modern gait.

Bipedalism – walking on two feet – is one of the defining characteristics of being human, and scientists have debated for years how it came about.

So, in the latest attempt to find an explanation, researchers trained five chimpanzees to walk on a treadmill while wearing masks that allowed measurement of their oxygen consumption. The chimps were measured both while walking upright and while moving on their legs and knuckles.

That measurement of the energy needed to move around was compared with similar tests on humans, and the results are in this week’s online edition of Proceedings of the National Academy of Sciences.

It turns out that humans walking on two legs use only one-quarter of the energy that chimpanzees use while knuckle-walking on four limbs. And the chimps, on average, use as much energy using two legs as they did when they used all four limbs.

However, there was variability among chimpanzees in how much energy they used, and this difference corresponded to their different gaits and anatomy.

One of the chimps used less energy on two legs, one used about the same and the others used more, said David Raichlen, assistant professor of anthropology at the University of Arizona. “What we were surprised at was the variation,” he said in a phone interview. “That was pretty exciting, because when you talk about how evolution works, variation is the bottom line; without variation, there is no evolution.”

If an individual can save energy moving around and hunting and spend more of it on reproduction, “that’s how you end up getting new species,” he said.

Among the other explanations for walking upright have been the need to use the arms in food gathering, the need to use the upper limbs to bring food to a mate and offspring, and raising the body higher to dissipate heat in the breeze.

Florida’s state social services agency investigated Nikolas Cruz’s home life more than a year before police say he killed 17 people at his former high school, closing the inquiry after determining that his “final level of risk is low,” despite learning that the teenager had behavioral struggles and was planning to buy a gun, according to an investigative report.